The invention relates to digital video processing, and more particularly, to a method and apparatus for scaling DVD subpicture data.
Displaying typical television/video source data on displays with different resolutions is well known. For example, video signals such as television broadcasts signals, cable television signals, satellite television signals, VCR signals, and DVD (which initials have been attributed to represent “digital versatile disc” and “digital video disc”) signals can be displayed on computer displays or televisions. Television also has different standards such as the National Television System Committee (NTSC) and the Phase Alternation Line (PAL) standards. The NTSC standard has a resolution of 720 pixels by 480 pixels whereas the PAL standard has a vertical resolution of 720 pixels by 576 pixels. Therefore, when converting between the NTSC and PAL display standards, or from one of these standards to a computer display standard, the television video signals must be scaled.
There are a variety of ways in which video data may be scaled. For example, when video data is to be upscaled (i.e., the target display is larger than the source display), pixel information may be repeated in several pixel locations to fill in the extra pixels of the upscaled display. Alternatively, pixel information may be blended within an area to fill in additional pixel locations that result from upscaling. For downscaling, the pixel information at a given pixel location is a result of merging pixel information of surrounding pixel locations of the source display. The merging may be a blending, averaging, selecting one of the pixel as a representative pixel, or combination thereof.
When video data is scaled, subpicture data needs to be scaled as well. Due to a small number of lines usually allocated to displaying subpicture data, it is important that the subpicture data be scaled properly.
Please refer to FIG. 1. FIG. 1 illustrates a subpicture unit 10 containing subpicture data. The subpicture unit 10 includes top field run-length data 12, bottom field run-length data 14, and SubPicture Display Control SeQuence (SPDCSQ) command data 16. Among other things, the SPDCSQ command data 16 indicates starting and stopping locations of subpicture data stored in the top and bottom run-length data 12 and 14, and indicates the starting and stopping time of the subpicture data to be displayed.
Please refer to FIG. 2. FIG. 2 illustrates a conventional subpicture scaling process. Non-scaled top and bottom subpicture data 20 and 22 is transformed into scaled top and bottom subpicture data 24 and 26 through a downscaling operation. As an illustrative example, FIG. 2 shows a downscaling operation in which one line is removed for every three lines of subpicture data. That is, every third line present in each of the non-scaled top subpicture data 20 and the non-scaled bottom subpicture data 22 is removed to create the scaled top and bottom subpicture data 24 and 26. In FIG. 2, the first five lines of the non-scaled top subpicture data 20 are labeled as T0, T1, T2, T3, and T4, and the first five lines of the non-scaled bottom subpicture data 22 are labeled as B0, B1, B2, B3, and B4. Since every third line is dropped from each of the top and bottom subpicture data, the lines T2 and B2 are shaded for indicating that these lines are removed from the scaled top and bottom subpicture data 24 and 26, respectively.
Please refer to FIG. 3. FIG. 3 illustrates the effects of subpicture scaling when displaying subpicture data in interlaced form. If the non-scaled top and bottom subpicture data 20 and 22 were to be interlaced and displayed in non-scaled form, the result would be the interlaced non-scaled subpicture data 30 shown in FIG. 3. However, if the scaled top and bottom subpicture data 24 and 26 were displayed in interlaced form, the result would be the interlaced scaled subpicture data 32. As is apparent in FIG. 3, since the scaling operation is performed on the non-scaled top subpicture data 20 and the non-scaled bottom subpicture data 22 separately, two adjacent lines of the interlaced non-scaled subpicture data 30 are removed. Because the interlaced scaled subpicture data 32 contains adjacent lines that are removed, the quality of the subpicture data may be severely degraded. For instance, if the letter “A” is to be displayed in the subpicture data, and two adjacent lines are removed, the effect could be the removal of the horizontal bar “-” in the letter “A”. In this case, the scaled version of the letter “A” would look something like a carrot top “^”.
In short, the conventional method for scaling subpicture data by scaling the top data and the bottom data separately has a negative effect on the quality of the subpicture. In addition to the problem noted above, use of the conventional scaling method may cause the subpicture to flicker and to become difficult to read.